1. Field of the Invention
The present invention concerns a method and a device for phase-sensitive flow measurement of a volume segment of an examination subject, using a magnetic resonance system.
2. Description of the Prior Art
In a phase-sensitive flow measurement (data acquisition) in a magnetic resonance (MR) system (for example a phase contrast flow measurement), additional bipolar gradient trains are superimposed on the gradient configuration required for imaging, in order to impress a gradient moment of the 1st order on the MR signal of a fluid that is to be detected. Flow coding is thereby achieved. The lower the flow velocities that are to be measured, the larger the moments that must be generated with the magnetic resonance system by the bipolar gradients. Under the additional boundary condition of an optimally short measurement duration, very strong gradient pulses must be used, which lead to eddy current fields within the volume segment to be measured or the slice to be measured. The acquired flow velocity information thus is disadvantageously adulterated.
For measurement of a flow within a vessel, the slices to be measured are typically aligned so as to be perpendicular to the longitudinal direction of this vessel. A flow coding gradient direction therefore also lies perpendicular to this longitudinal direction of the vessel. Since the longitudinal direction of the vessel coincides with one of the three basic spatial directions or gradient axes of the magnetic resonance system only in the rarest cases, the bipolar gradients for flow coding typically have physical components with regard to all three gradient axes or basic spatial directions. This individual slice alignment, aligned on the vessel to be examined, therefore leads to the situation that the exposure fields likewise occur individually in the acquired flow images as background phase effects with different directions and dimensions for every measurement. The exposure effects are thereby even more significantly intensified by the coupling of the different gradient directions, this coupling being dependent on the slice direction.
The following possibilities to correct background phases in phase-sensitive flow measurement that occur due to the eddy current effects exist according to the prior art.
The background phases can be corrected by reference measurements of a stationary phantom. For these measurements, however, protocol step repetitions with exactly identical settings are required for each patient, so the time duration to implement the phase-sensitive flow measurement is more than doubled. See “Baseline Correction of Phase Contrast Images Improves Quantification of Blood Flow in the Great Vessels” by A. Chernobelsky et al., Journal of Cardiovascular Magnetic Resonance 2007, Vol. 9, No. 4, Pages 681-685.
Another approach is to measure and store reference measurements of the phantom in advance for all conceivable parameter settings, in particular for all possible slice orientations. The data measured for a patient are then corrected with the aid of that phantom data set which best matches the parameter setting which is used for the current measurement protocol of the patient. See “Temporal stability of the background velocity error supports automated correction of flow measurements”, P. D. Gatehouse et al., Proc. Intl. Soc. Mag. Reson. Med. 16 (2008), Page 391.
According to a further approach, the background phase is corrected by means of an intrinsic reference. For this a hypersurface is adapted to stationary pixels, wherein it is assumed that this hypersurface sufficiently approximates the background phase contributing to the flow signal at the location of the vessel so that the errors occurring due to the background phase can be corrected. See “Correction of Phase Offset Errors in Main Pulmonary Artery Flow Quantification”, by J.-W. Lankhaar et al., JOURNAL OF MAGNETIC RESONANCE IMAGING 22 (2005), Pages 73-79.
Most approaches known according to the prior art are based on a subsequent correction of the background phase using reference information. Approaches are also known to model the eddy current effects or to detect them independent of the imaging itself.